1. Field of the Invention
The present invention relates generally to the field of optical telecommunications, and, more specifically, to optical devices such as optical add/drop multiplexers (OADMs).
2. Description of the Related Art
In state-of-the-art, optical wavelength-division-multiplexed (WDM) communication systems, multiple, individually-data-modulated wavelengths of light are multiplexed into an optical fiber and routed to a remote location via ring or mesh optical networks. Along the path from source to destination, optical devices, such as OADMs, are commonly employed to modify the contents, and in some cases the quality, of the multiplex. Such modification includes grooming the multiplex by adding, dropping, replacing, and/or frequency-translating data-carrying wavelengths of light within the multiplex.
At the input or “drop” side of an OADM, an input WDM signal is typically split (e.g., using a power splitter) into a pass leg and a drop leg. The drop leg typically feeds a receiver that is configured to receive, detect, and demodulate one of the data-modulated wavelengths λd of the input multiplex. The pass leg is typically fed to a wavelength blocker that is configured to block the dropped wavelength λd, and pass the rest of the wavelengths in the multiplex to the “add” side of the OADM. Alternatively, the functionality of power splitting and wavelength blocking can be integrated into a WDM demultiplexing filter.
At the add side of the OADM, an optical transmitter is configured to modulate a locally supplied data stream onto an optical carrier of the same wavelength λd that was dropped from the drop side of the OADM. This modulated wavelength from the transmitter is then combined with the wavelengths of the multiplex that were passed to the add side from the drop side to form a new multiplex that is output from the OADM.
The transmitter in the OADM is commonly of the externally modulated laser variety. Such a transmitter includes a fixed-wavelength or a tunable laser coupled to a modulator (e.g., a Mach-Zehnder modulator) whose modulation is controlled by a properly coded version of the locally supplied data stream.
The receiver is typically of the direct-detection variety. Such receivers generally include a pin-type or avalanche-type photodiode coupled to an amplifier, filtering electronics, and a sampling/decision circuit.